Copolymers of maleic anhydride and N-vinyl2-pyrrolidone (referred to herein as vinylpyrrolidone) are well known and are described for instance in U.S. Pat. No. 2,676,949 the disclosure of which is incorporated herein by reference. Such copolymers have a number of uses but are especially useful in removing rust from metal surfaces as described in U.S. Pat. No. 4,424,079.
Copolymers of vinylpyrrolidone and maleic anhydride can be made in a variety of molecular weights but it has previously been necessary to use varying reaction conditions to make such copolymer of varying molecular weights. In general, more severe reaction conditions, i.e. higher temperatures or pressures produce copolymer of relatively lower molecular weight while less severe conditions produce copolymer of relatively higher molecular weight.